Size of an object from a distance of the camera
Notes towards an answer.
The size of the final output image (in pixels) depends on the resolution settings for the project.
There is no default size for pixels when images are used as textures, the size of the image depends on the dimensions of the object, and how the textures are mapped as part of the material.
The size of objects in the camera view depends on three different variables:

The dimensions of the object.

The camera lens (or field of view in degrees).

The distance between the object and camera.


Answer.
With the information above you can then use Trigonometry(https://en.wikipedia.org/wiki/Trigonometry) to calculate camera placement, or lens size, or the dimensions of the object so that the image fills the screen.


End of answer.

For a lazy way to have an image fill the screen read on:
When you import an &quot;image as plane&quot; you can determine the size. By default blender will import an image as a plane that measures 1 unit in height.

But if you open the settings, by pressing N or pressing the gear icon on the top right, you can change the settings to Camera Relative.
That will allow you choose to have the image be at a scale that fills the current camera view. Select Fill or Fit.

Fill will fill the camera frame. (if the image is a different aspect ratio than that of the camera then the image might spill out of the frame)
Fit will fit the entire image within the camera frame (if the image is a different aspect ratio than that of the camera the image will have some empty space around it)
If the camera view and the images are the same aspect ratio, then you can use either one of those choices.